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Chemistry - Valence and Chemical Formulae

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Information about Chemistry - Valence and Chemical Formulae
Education

Published on May 15, 2013

Author: AlexanderZhongPing

Source: slideshare.net

Description

This presentation will discuss on valencies and how to deduce the chemical formulae of a pure substance.
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This presentation was produced by the Sciencedepartment of Temasek Secondary School.Redistribution or reproduction of this resource isprohibited by copyright regulations. Thisresource should be used for educationalpurposes.

Acids and AlkalisValency & Chemical Formulae

Today’s Lesson The Meaning of Valency Naming Compounds Writing Chemical Formula

Valency Elements react in order to achieve noble gaselectronic configuration. E.g. chlorine with an electronic configuration of 2.8.7gains one electron to achieve an electronicconfiguration of 2.8.8.

Valency To write the chemical formula of a compound, we mustknow the valency of the elements or ions involved. Valency is the number of electrons that an atomneeds to gain or lose in order to achieve noble gaselectronic configuration. E.g. chlorine has a valency of 1, as it needs to gain 1electron to achieve noble gas electronic configuration.

GroupnumberElementElectronicstructureNo of electrons to gain or lose to achievethe noble gas electronic configurationIon formed ValencyI Li 2.1 Lose 1 electron Li+1I Na 2.8.1 Lose 1 electron Na+1I K 2.8.8.1 Lose 1 electron K+1II Be 2.2 Lose 2 electrons Be2+2II Mg 2.8.2 Lose 2 electrons Mg2+2

GroupnumberElementElectronicstructureNo of electrons to gain or lose to achievethe noble gas electronic configurationIon formed ValencyII Ca 2.8.8.2 Lose 2 electrons Ca2+2III B 2.3 Lose 3 electrons B3+3III Al 2.8.3 Lose 3 electrons Al3+3V N 2.5 Gain 3 electrons N3-3V P 2.8.5 Gain 3 electrons P3-3

GroupnumberElementElectronicstructureNo of electrons to gain or lose to achievethe noble gas electronic configurationIon formed ValencyVI O2.6 Gain 2 electronsO2- 2VI S 2.8.6 Gain 2 electrons S2-2VII F2.7 Gain 1 electronF- 1VII Cl 2.8.7 Gain 1 electron Cl-1

Polyatomic Ions There are some ions that are made up of more thanone atom. They are called polyatomic ions.Some common polyatomic ionsPositive Ion Valency Negative Ion ValencyAmmonium ion, NH4+1 Nitrate ion, NO3-1Hydroxide ion, OH-1Carbonate ion, CO32-2Sulfate ion, SO42-2

Chemical Formula The chemical formula of a substance shows the numberof atoms and the elements that make up the compound.CompoundChemicalformulaNumber of atoms and elementsSodium hydroxide NaOH 1 Na, 1 O and 1 H atomsCopper sulfate CuSO4 1 Cu, 1 S and 4 O atomsAmmonia gas NH3 1 N and 3 H atomsCalcium carbonate CaCO3 1 Ca, 1 C and 3 O atomsAluminium fluoride AlF3 1 Al and 3 F atoms

Naming CompoundsWhen naming an ionic compound that contains onlytwo elements:Metallic element comes before non-metallic elementNon-metallic elements end with ‘ide’

Naming CompoundsElements in an ionic compound Chemical nameCalcium, chlorineCalcium chloride(metal) (non-metal)Lithium, fluorine Lithium fluorideMagnesium, oxygen Magnesium oxidePotassium, sulfur Potassium sulfide

Naming Compounds A polyatomic ion will not change its name aftercombining with other elements.Examples: Sodium carbonate Potassium sulfate Calcium hydroxide Ammonium chloride

Your turn to practise!

ActivityChemical formula of compounds containing the followingsymbols/ groups are called:Ion Name ExampleF-fluoride LiF, lithium fluorideCl-chloride NaCl, sodium chlorideBr-bromide KBr, potassium bromideI-iodide MgI2, magnesium iodide

ActivityChemical formula of compounds containing the followingsymbols/ groups are called:Ion Name ExampleO2-oxide MgO, magnesium oxideN3-nitride Na3N, sodium nitrideP3-phosphide K3P, potassium phosphideNH4+ammonium NH4Cl, ammonium chloride

ActivityChemical formula of compounds containing the followingsymbols/ groups are called:Ion Name Example OH- hydroxide KOH, potassium hydroxide NO3- nitrate NaNO3, sodium nitrate CO32- carbonate MgCO3, magnesium carbonate SO42- sulfate K2SO4, potassium sulfate PO43-phosphate Na3PO4, sodium phosphate

Writing Chemical FormulaExample 1: Sodium chloride             Steps Sodium chloride123Write the symbols of the elements, theirrespective valencies below the symbols,and criss-cross.Na Cl1 1Attach the valency as a subscript.Na1Cl1Simplify the formula.NaCl

Writing Chemical FormulaExample 2: Zinc chloride             Steps Zinc chloride123Write the symbols of the elements, theirrespective valencies below the symbols,and criss-cross.Zn Cl2 1Attach the valency as a subscript.Zn1Cl2Simplify the formula.ZnCl2

Writing Chemical FormulaExample 3: Aluminium oxide             Steps Aluminium oxide123Write the symbols of the elements, theirrespective valencies below the symbols,and criss-cross.Al O3 2Attach the valency as a subscript.Al2O3Simplify the formula.Al2O3

Writing Chemical FormulaExample 4: Magnesium oxide             Steps Magnesium oxide123Write the symbols of the elements, theirrespective valencies below the symbols,and criss-cross.Mg O2 2Attach the valency as a subscript.Mg2O2Simplify the formula.MgO

Writing Chemical FormulaTAKE NOTE when simplifying chemical formulae:The subscript ‘1’ must be omittedReduce the ratio by a common factor if possible

Complete Exercise 1

Writing Chemical Formula For compounds that contain polyatomic ions, a bracketmust be used if a number is required.

Writing Chemical FormulaExample 5: CalciumnitrateSteps Calcium nitrate123Write the symbols of the elements, theirrespective valencies below the symbols,and criss-cross.Ca NO32 1Attach the valency as a subscript.Ca1(NO3)2Simplify the formula.Ca(NO3)2

Writing Chemical FormulaExample 6: SodiumcarbonateSteps Sodium carbonate123Write the symbols of the elements, theirrespective valencies below the symbols,and criss-cross.Na CO31 2Attach the valency as a subscript.Na2(CO3)1Simplify the formula.Na2CO3

Writing Chemical FormulaExample 7: AluminiumsulfateSteps Aluminium sulfate123Write the symbols of the elements, theirrespective valencies below the symbols,and criss-cross.Al SO43 2Attach the valency as a subscript.Al2(SO4)3Simplify the formula.Al2(SO4)3

Writing Chemical FormulaExample 8: AmmoniumchlorideSteps Ammonium chloride123Write the symbols of the elements, theirrespective valencies below the symbols,and criss-cross.NH4 Cl1 1Attach the valency as a subscript.(NH4)1Cl1Simplify the formula.NH4Cl

CompleteExercise 2

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